A hearing instrument usually consists of a shell which is shaped to fit inconspicuously into an ear canal or behind an ear. The shell generally accommodates a microphone, a receiver (speaker), an amplifier or a DSP, and a battery. The microphone is typically of the electret condenser type, and the DSP or amplifier is ordinarily a separate component which must be hardwired to the other components of the hearing instrument. Producing a hearing instrument requires balancing several competing considerations including the volume and shape of the shell, cost, and the desired functions to be incorporated into the hearing instrument.
Fabrication of precision-machined electret condenser microphones (ECMs) is a relatively time- and labor-intensive process, and variations in uniformity and reliability pose challenges relative to the design and performance of microphones or transducer assemblies. For example, directional microphone applications using a matched pair require precise tolerances so as to avoid the undesired influences of sensitivity mismatching. The use of wires to connect components together inside a hearing instrument poses further challenges. Connecting wires is a labor-intensive process, and they are susceptible to electromagnetic interference which can adversely affect performance.
Many of the challenges posed by ECMs have been overcome with the advent of silicon-based transducers which are fabricated using microelectromechanical systems (MEMS) technology. A batch of these transducers can be fabricated on a single wafer, increasing uniformity and lowering production costs. They can also be surface mounted to a substrate by standard solder reflow techniques, thereby obviating the use of wires conventionally used with ECMs.
Multiple microphones can be arrayed to provide directionality or adaptive beam steering. In a conventional microphone array, such as a matched pair, each ECM in the array must be precision machined so as to have nearly identical sensitivity and/or response characteristics for optimal performance. Moreover, each additional ECM consumes more space, which poses yet additional design challenges.
Silicon-based transducers offer numerous advantages and characteristics including a small space consumption, uniformity, and reliability, for example. The present invention is directed to exploiting these advantages in hearing instruments and other applications.